Halt phosphatase inhibitor cocktail preserves phosphorylation of proteins in cell and tissue lysates. The cocktail contains a mixture of four phosphatase inhibitors of broad specificity: sodium fluoride, sodium orthovanadate, sodium pyrophosphate and β-glycerophosphate. Phosphoproteins are protected from both serine/threonine phosphatases and protein tyrosine phosphatases. The cocktail is compatible with standard protein assays, and can be stored at 2 C to 8 C.